Plasma-electrolytic deposition of protective coatings on light metals is a well-established process in the prior art, especially the deposition of oxides/hydroxides of the elements Si, Zr and/or Ti on aluminum substrates.
WO 03/029529 A1 discloses a method for the plasma-electrolytic deposition from aqueous electrolytes that comprise fluorometallates of the elements Si, Zr and/or Ti. The aluminum or magnesium substrate acts as an anode in the process described therein and rapid formation of a protective coating is reported. The protective coatings are attained via pulse direct current or alternating current with a frequency ranging from 10-1000 Hertz and a current density in the range from 1-3 A/dm2. The protective coatings exhibit good corrosion-, heat-, and abrasion-resistance.
However, when applying the before-mentioned plasma-electrolytic deposition method the appearance of white spots at extended times of deposition that are aimed to yield protective coating thicknesses of above 15 microns is critical. These white spots are defects in the protective coating at which corrosive attack of the beneath substrate is initiated. The appearance of white spots during the layer built up thereby also factually limits the coating thickness for which suitable corrosion resistance can be attained. In addition, a plasma-electrolytic deposition of the prior art usually reaches relatively quickly an equilibrium of corrosion rate and deposition rate so that coating thicknesses above 15 μm can only be obtained under harsh electrical conditions to uphold a voltage drop across the protective coating that allows a sustained plasma at the substrate to be further coated. These observations are especially true for the plasma-electrolytic deposition of protective coatings on the substrate aluminum. Said substrate being of outstanding economic importance due to a still increasing number of applications to which aluminum articles are essential, such as in light weight constructions being an important technology driver in automotive industry.
The objective of the underlying invention therefore consists in providing a method for the plasma-electrolytic deposition of an inorganic protective coating on aluminum-containing metallic material that enables economically reasonable deposition rates even at coating thicknesses above 15 μm while attaining protective coatings with less defects prone to corrosion and a superior coating hardness.